The invention relates to an hydraulic valve arrangement. In particular, but not exclusively, the invention relates to a valve arrangement which is suitable for use in controlling the flow of fuel to the ignitor jets of a gas turbine engine.
In a multi-stage gas turbine engine, the arrangement of burners commonly includes a set of pilot burners, which are on at all times when the engine is running, and one or more sets of main burners. During the engine start-up sequence, a set of ignitor jets is used to initiate firing of the pilot burners. The pilot burners ignite the main burners which are fired in stages as thrust demand increases.
Within the aircraft fuel system for a gas turbine engine, fuel is pumped from the main fuel storage tanks (usually within the aircraft wings) by means of a first, electrically operated xe2x80x98lift pumpxe2x80x99. The lift pump provides a fuel input to a xe2x80x98low pressurexe2x80x99 engine driven pump which, in turn, supplies a xe2x80x98high pressurexe2x80x99 engine driven pump, usually in the form of a gear pump. The gear pump provides a supply of fuel, pressurised to a relatively high level, for fuelling the engine.
In known fuel supply systems for gas turbine engines, a first Pressure Raising Shut-Off Valve (PRSOV) is provided in the supply path to the pilot burners to enable the flow of fuel to these burners to be shut off. A second PRSOV is provided in the supply path to the main burners for the same purpose. The flow of fuel from the ignitor jets is tapped off from the pilot burner supply path at a position upstream of the first PRSOV and a control valve is provided to control the flow of fuel to the ignitor jets. It is undesirable to provide a further PRSOV in the tapped off supply path to the ignitor jets due to the cost and weight disadvantages.
Upon engine start-up, when fuel within the main sets of burners is ignited and the engine has fully started, there are benefits in maintaining a relatively low rate of flow of fuel to the ignitor jets. A low rate of flow of fuel to the ignitor jets provides a cooling function for the burner, and serves to prevent carbonisation within the burner and the fuel supply pipes which may otherwise cause blockage of the ignitor jets. Additionally, should the engine flame go out in one of the main burners, the permanent supply of re-ignition fuel to the ignitor jets enables immediate re-lighting.
When the aircraft is on the ground and the engine is shutdown, it is important that the flow of fuel to the ignitor jets is terminated. There are circumstances in which it is desirable for the lift pump to be operated when the aircraft is grounded, for example for test purposes or if the aircraft is only landed for a short period of time, and in such circumstances a slightly pressurised fuel flow is maintained in the system. No such fuel is able to leak into the main burners or the pilot burners due to the provision of the first and second PRSOVs which are biased closed by a relatively strong spring force. However, the spring for the ignitor jet control valve provides a weaker biasing force, and the valve is configured such that any slightly pressurised fuel within the system will be sufficient to overcome the relatively weak spring force, thereby causing fuel to leak to the ignitor jets. Any leakage of fuel into the engine when the aircraft is landed and engine operation is halted is highly undesirable as it can result in the production of smoke within the engine at the next engine start-up and, in extreme cases, may result in an explosion. Any fuel leakage from the engine also presents an environmental hazard and increases the risk of ground fire. Furthermore, when the engine is still hot after engine shutdown, a small flow of fuel through the ignitor jets may, if repeated over many occasions, result in blockage of the jets through carbonisation of the leakage fuel.
The aforementioned fuel leakage problem may be overcome through use of a further PRSOV in the flow path to the ignitor jets, but this solution has prohibitive cost and weight implications.
It is an object of the present invention to provide a valve arrangement which enables the above mentioned disadvantages to be overcome.
According to the present invention, there is provided an hydraulic valve arrangement including a fluid pressure actuable valve which is actuable between a first position in which fluid flow through the valve is prevented and a second position in which fluid flow through the valve is permitted at a first rate, and a second valve which is actuable between an open state, in which fuel is able to flow through the second valve at a second rate, and a closed state in which there is no flow through the second valve, the fluid pressure actuable valve being arranged such that, upon initial actuation of the second valve to its open state, the fluid pressure actuable valve is caused to move into the second position, and whereby the fluid pressure actuable valve remains in the second position upon subsequent switching of the second valve between the open and closed states, thereby to permit the rate of flow of fuel through the hydraulic valve arrangement to be varied by switching the second valve.
The invention is particularly advantageous when employed in an aircraft fuel system for a gas turbine engine, where the hydraulic valve arrangement is used to vary the rate of flow of fuel to the ignitor jets of the engine between the relatively high rate required upon engine start-up and the lower rate required when the engine burners are fully operational. The invention also provides the advantage that, when the aircraft has landed and any flow of fuel to the ignitor jets is undesirable, both the fluid pressure actuable valve and the second valve can be adequately closed to prevent fuel leakage to the engine.
In a preferred embodiment, the fluid pressure actuable valve includes a resiliently biased piston member which is moveable between a first position, in which fluid flow through the fluid pressure actuable valve is prevented, and a second position in which fluid flow through the fluid pressure actuable valve occurs at a first, relatively restricted rate. The piston member is preferably moveable within a bore provided in a valve housing.
Preferably, the fluid pressure actuable valve includes first and second control chambers for fluid, whereby the position of the piston member is controlled by controlling fluid pressure in at least one of the control chambers.
The fluid pressure actuable valve is conveniently arranged within a primary flow path for fluid, which is provided with a first restriction through which fluid flows at the relatively restricted rate when said valve is in its second position.
In one embodiment, the second valve comprises a by-pass valve member, for example a spherical valve member, which is engageable with a seating to control fluid flow through a by-pass flow path, whereby when the by-pass valve member is lifted from its seating fluid is able to flow through the by-pass flow path at a second, relatively high rate.
Preferably, the fluid pressure actuable valve and the second valve are arranged such that, upon initial actuation of the second valve to the open state, fluid flows through the by-pass flow path into the second control chamber, thereby applying a force to the piston member to urge the piston member into the second position.
Conveniently, the valve member is actuable by means of an electromagnetic actuator.
When the hydraulic valve arrangement is employed in an engine fuel supply system, once the fluid pressure actuable valve has been moved into its second position upon actuation of the second valve, the hydraulic valve arrangement permits the rate of flow of fuel to be varied simply by switching the second valve between its open and closed states. The hydraulic valve arrangement is configured such that, during subsequent switching of the second valve, the fluid pressure actuable valve remains latched in its second position (i.e. a latched open state). Thus, in use, once the engine has been fully started and only a relatively low rate, cooling flow of fuel to the ignitor jets is required, the flow rate can be switched by moving the second valve to its closed state, in which case fuel is only able to flow to the ignitor jets at a relatively low rate. If engine xe2x80x98flame-outxe2x80x99 occurs, such that a higher flow rate of fuel to the ignitor jets is required to re-ignite the burners, this can be achieved near instantaneously by switching the second valve to its open state, thereby permitting fuel to by-pass the restriction in the primary/ignitor jet flow path by flowing through the relatively unrestricted, by-pass flow path.
Upon engine shut-down, a reduction in fuel pressure within the primary supply path when the supply system pumps are de-activated results in the fluid pressure actuable valve being unlatched (i.e. moving into its first position) to terminate the flow of fuel through the primary flow path. If the second valve is switched to its closed state, flow is also terminated through the by-pass flow path. With both valves closed, fuel leakage to the engine is avoided.
Conveniently, the first control chamber of the fluid pressure actuable valve is supplied with fuel at low pressure from a low pressure fuel reservoir, the pressure of fuel due to fuel within the first control chamber acting in combination with a spring force to urge the piston member towards the first position.
It is important that the flow capacity of the ignitor jets, or any restriction to fuel flow downstream of the point of communication between the primary and by-pass flow paths, is less than the flow capacity through the second valve. In this way, fuel pressure in the connection to the ignitor jets will cause fuel to flow via the by-pass flow path into the second control chamber upon actuation of the second valve. This ensures the piston member is urged away from its first position, into its second position, and remains xe2x80x98latchedxe2x80x99 in its second position during subsequent switching of the second valve.